Magnetic Resonance Imaging (“MRI”) systems require the generation of an extremely strong magnetic field, which is generally measured in units referred to as “Tesla”. (One Tesla=10,000 Gauss.) In order to achieve a magnetic field of this strength, it is generally necessary to employ superconducting magnets, which include coil windings that are cooled to temperatures on the order of a few degrees above absolute zero, using liquid helium as a coolant in the form of a cryogenic bath. Aside from the difficulties posed by the intense magnetic field itself, the handling of large quantities of such extremely cold liquid helium poses certain inherent difficulties.
One such difficulty is associated with the quenching of the superconducting coils of the magnet. “Quenching” in this context refers to a sudden loss of superconductivity in the wire that makes up the superconducting coils. As the coils start to exhibit normal resistive behavior, they heat up, causing the process to accelerate, so that the liquid helium “boils” off rapidly, releasing the magnet's stored energy in a process that can become somewhat violent. Moreover, the large volume (thousands of cubic meters) of evaporated liquid helium, which is released rapidly via a quench line remains extremely cold, and can cause injury, including “cold burns”, to anyone who comes into contact with it. Asphyxiation is also a hazard.
Quenching may be performed intentionally, such as when it becomes necessary to shut down the magnetic field in order to prevent personnel or patient injury, or it may occur spontaneously due to a failure in the magnet system itself or an external influence. In either case, it is apparent that the manner in which the resulting discharge of evaporated helium gas is guided and vented to the exterior is extremely important. In particular, the design of the so-called “quench line” is significant, and must be configured so as to minimize the risk that people, animals or damageable objects will come into direct contact with the gas discharge. Moreover, it is also essential that the quench line be capable at all times of venting the evaporated helium at a rate that accommodates the rapid boiling in the cryogenic unit. If, for example, the quench line is inadequate or becomes constricted or clogged, a particularly dangerous situation can result. One such possibility is that moisture accumulates in the quench line, blocking it and causing helium gas to be vented into the examination area, which can result in asphyxiation.
Mobile MRI systems of the type mentioned previously are subject to all of the considerations described above, and in addition present their own unique design problems as well. For example, there is an increased risk of a spontaneous quench of the cryogenic cooling system due to “jostling” of the mobile MRI device between field locations. In addition to mechanical vibrations, systems are exposed to varying electromagnetic environments during transport which can also induce a quench. In addition, the necessity for movement of the trailer along routes populated by other vehicles is also of concern. For example, if the trailer is in a line of traffic, with a bus immediately behind, passengers at the front of the bus on the upper floor might be at risk of personal injury from cold gas in the event of a magnet quench. Similar risks have been identified to personnel working on ladders or raised platforms behind a mobile MRI system which is installed at a site. In order to address safety risks to service personnel, known mobile MRI systems have been designed to be refilled with liquid helium by service personnel located outside and to the rear of the trailer/housing, beneath the quench line exit.
To deal with these considerations, the exit of the quench line for mobile MRI systems must meet the following criteria:                Provide a safe means of venting helium gas from the helium vessel under magnet service and quench conditions;        Not generate a significant pressure drop, or restrict the gas flow;        Inhibit the ingress of rain water, wind-borne debris and wildlife;        Allow any water in the quench line to drain away;        Be compatible with maximum trailer dimensions and national regulations regarding appendages to the exterior of the trailer;        Minimize cost to manufacture; and        Minimize the requirement for internal space within the trailer.        
Conventional horizontal quench line exits do not direct quench flow gas away from pedestrians or bus passengers. During magnet depressurization and filling, air cooled by the released helium gas could impinge on service personnel beneath the exit grill. If the inner surface of the quench line exit is not angled downwards, condensation will reside in the quench line, with serious consequences if this migrates to the quench valve assembly.
Covers have been fitted to the exterior of horizontal quench line exit grills on previous MRI mobile installations, primarily to prohibit the ingress of rainwater. These designs were not favored by trailer manufacturers since appendages to the trailer are limited by road regulations (maximum trailer width), and compact cover designs can lead to large pressure drops for the quench gas flow. Hinged covers over exit grills are not permitted for any MRI installations (mobile or static) within the guidelines provided by Siemens Magnet Technology for quench line design (830-105HB2).